1. Field of the Invention
The present invention relates to a display device in which the directivity of the display is increased with respect to a specific direction, to a terminal device provided with this display device, and to a light source device and optical member incorporated into the aforementioned display device.
2. Description of the Related Art
Due to recent advances in technology, display panels have been deployed in a range of devices that includes monitors, television sets, and other large terminal devices; notebook-type personal computers, cash dispensers, vending machines, and other mid-sized terminal devices; and personal TVs, PDAs (Personal Digital Assistance: personal information terminal), mobile telephones, mobile gaming devices, and other small terminal devices that are used in a variety of locations. Because of their thin profile, light weight, small size, low energy consumption, and other advantages, display devices that use liquid crystals are particularly common in terminal devices.
Among these terminal devices, small-to-medium-sized terminal devices are characteristically used not only in closed rooms under tight security, but also in public places. It then becomes important to keep displays of private information and confidential information from being viewed by a third party. Particularly in recent years, occasions where private information and confidential information are displayed have increased in conjunction with the development of terminal devices, and demand for eavesdropping prevention techniques is increasing. A display device in which the display can be viewed only by a user positioned in front or in another specific direction, and eavesdropping from other directions is prevented by narrowing the range of angles in which the display is visible has been proposed together with an eavesdropping-preventing optical member applied to this display device (see Japanese Laid-Open Patent Application 2003-131202, for example: hereinafter referred to as Prior Art 1).
FIG. 22 is a sectional view showing the anti-eavesdropping device disclosed in Prior Art 1. This anti-eavesdropping device is affixed to the display surface of the display device and used. In this conventional anti-eavesdropping device as shown in FIG. 22, a thin anti-glare layer 1101 is provided, and a thin adhesive layer 1110 having high translucency is layered on and attached to the back surface of this anti-glare layer 1101. A silicone adhesive layer 1120 is also provided to the surface of the anti-glare layer 1101, and a translucent thin translucent layer 1130 is integrally bonded and layered via the silicone adhesive layer 1120. The anti-glare layer 1101, the adhesive layer 1110, and the translucent layer 1130 are each in the form of a flexible sheet or film. The surface of the adhesive layer 1110 on the opposite side from the anti-glare layer 1101 is a smooth, translucent attachment surface 1111 having a mirror finish, and is an attaching surface that can be detachably affixed to the display surface of the liquid crystal display (not shown in the drawing) of an information display device.
The anti-glare layer 1101 is formed by integrating a plurality of transparent silicone rubber sheets 1102 and a plurality of colored silicone rubber sheets 1103 arranged in alternating fashion in the direction parallel to the surface of the anti-glare layer 1101. The adjoining surfaces of the transparent silicone rubber sheets 1102 and the colored silicone rubber sheets 1103 are parallel to each other. The width, specifically, the thickness in the lateral direction of FIG. 22 of the transparent silicone rubber sheets 1102 and colored silicone rubber sheets 1103, is selected with consideration for the fact that the transparency and parallel light transmittance are determined by the ratio of the width of the transparent silicone rubber sheets 1102 to the width of the colored silicone rubber sheets 1103, and for the fact that the range of viewing angles is determined by the refractive index and width of the transparent silicone rubber sheets 1102 and the overall thickness of the anti-eavesdropping device. In Prior Art 1, the width of the transparent silicone rubber sheets 1102 is described as being 100 to 200 μm, for example, and preferably 120 to 150 μm; and the width of the colored silicone rubber sheets 1103 is described as being 10 to 50 μm, for example, and preferably 10 to 30 μm. According to Prior Art 1, by setting such values for the widths of the transparent silicone rubber sheets 1102 and colored silicone rubber sheets 1103, the anti-glare layer 1101 can be endowed with a parallel ray transmittance of approximately 80% or higher with a maximum of 85% or higher, and a visibility range of 90 to 120 degrees. According to Prior Art 1, the thickness of the anti-eavesdropping device is set to about 0.15 to 0.5 mm with consideration for the angle range of visibility, translucency, and handling, and is more preferably set to about 0.15 to 0.3 mm to enable attachment to the liquid crystal display of a small, thin mobile telephone or the like.
Affixing this type of anti-eavesdropping device to the display surface of a display device prevents light from exiting from the anti-eavesdropping device since light that is incident in a direction tilted with respect to the anti-eavesdropping device is absorbed by the colored silicone rubbers sheets forming a louver. Specifically, the anti-glare layer 1101 of the anti-eavesdropping device used for an information display demonstrates anti-eavesdropping effects. It is thereby impossible or extremely difficult for a third party present beside the user to see from the side or read the various types of information displayed when the anti-eavesdropping device is mounted on the liquid crystal display of the information display. Accordingly, since the information displayed on the information display is not leaked to a third party, the user of the information display can monitor and transmit information comfortably without worrying about eavesdropping.
However, the anti-eavesdropping device described in Prior Art 1 has such problems as those described below. Specifically, when the anti-eavesdropping device described in Prior Art 1 is attached to the display device, moiré occurs to a significant degree between the pixels of the display device and the colored silicone rubber sheets constituting the anti-eavesdropping device, and the display quality is severely reduced.
Techniques for suppressing moiré have been developed in the past to remedy this problem (see Japanese Patent No. 2622762, for example: hereinafter referred to as Prior Art 2). FIG. 23 is a diagram showing a conventional raster display device provided with a light control film disclosed in Prior Art 2; FIG. 24 is a top view showing the positioning of the light control film with respect to the display surface of the display device; and FIG. 25 is a graph showing the relationship between angle β and pitch p, wherein the angle β (degrees) between the raster of the display device and the stripes of the light control film is plotted on the horizontal axis, and the pitch p (mm) of the moiré stripes is plotted on the vertical axis.
As shown in FIG. 23, the raster display device 3102 described in Prior Art 2 is installed in an in-vehicle information display system, for example, and the in-vehicle information display system is composed of an automobile state detection device 3101, a CRT (Cathode Ray Tube) display device or other raster display device 3102 having a raster aligned so that the pitch is a, an information display controller 3103, a light control film 3104 for controlling the transmission direction of light, and an operating input device 3106. The light control film 3104 is attached to the display surface of the raster display device 3102. A user 3105 can see the raster display device 3102 through the light control film 3104.
The light control film 3104 controls the transmission direction of incident light, and has light-transmitting and light-blocking portions arranged in alternating stripes at a prescribed pitch therein. The light control film 3104 is reinforced by a glass plate. The light control film 3104 is also offset with respect to the raster display device 3102 so that the direction in which the stripes of the control film 3104 extend is tilted a prescribed angle β, for example, 10 degrees, with respect to the raster direction of the raster display device.
As shown in FIG. 24, a moiré bar whose pitch is p occurs at the intersection between the raster (indicated by straight line A) and the stripe (indicated by straight line B) when the display device 3102 and the light control film 3104 are viewed from the front. This moiré bar is indicated by the dotted line C. In this case, when the angle β is relatively small, the pitch p of the moiré bar can be computed using the Eq. 1 below, where a is the pitch of the raster, k is a coefficient, (a×k) is the pitch of the stripes, and B (degrees) is the angle formed by the extension direction of the raster and the extension direction of the stripes.
                    p        =                                                                        a                ⨯                k                                            cos                ⁢                                                                  ⁢                β                                                                                                                      (                                      tan                    ⁢                                                                                  ⁢                    β                                    )                                2                            +                                                (                                      1                    -                                          k                                              cos                        ⁢                                                                                                  ⁢                        β                                                                              )                                2                                                                        [                  Eq          .                                          ⁢          1                ]            
FIG. 25 is a diagram in which the abovementioned Eq. 1 is plotted. As shown in FIG. 25, the pitch p of the moiré bar decreases as the angle β increases, regardless of the size of the coefficient k. By making the pitch p of the moiré bar about the same or smaller than the pitch a of the raster, the moiré bar can be made less visible to the user. The angle β required to achieve this result is approximately 3 degrees or larger.
According to the description in Prior Art 2, a moiré bar is thus generated by the superposition of the light-blocking portions of the control film and the arrangement of the pixels in the image display device when a control film in which light-transmitting striped portions and light-blocking striped portions are arranged in alternating fashion and which is used to control the transmission direction of light is mounted for the purpose of preventing eavesdropping and the like on the display surface of an image display device in which a plurality of pixels are periodically arranged in two dimensions, and in which an arbitrary image is displayed. However, by tilting the extension direction of the control film stripes three degrees or more with respect to the arrangement direction of the pixels of the image display device, the pitch of the moiré bar can be made smaller than the arrangement pitch of the pixels, and the effect of moiré on the image can be reduced to a certain degree.
However, the above-described conventional technique has such problems as those described below. As described in Prior Art 2, moiré can be reduced to a certain degree by tilting the extension direction of the control film stripes with respect to the arrangement direction of the pixels of the image display device. However, moiré still occurs, and the moiré-reducing effects are inadequate. As described also in FIG. 3 of Prior Art 2, the period of the moiré increases particularly when the pitch of the opaque portions of the control film is near the pixel pitch of the display device. It is therefore impossible to adequately reduce moiré even when the control film is placed in a tilted position. In this case, when the tilt angle of the control film is increased in order to reduce moiré, the direction in which the light is restricted, specifically, the direction in which eavesdropping is prevented, is tilted from the horizontal direction, thus creating discomfort for the user.